Composite structural metal members with improved fracture toughness



Dec. 19, 1967 H. LEICHTER 3,359,083

COMPOSITE STRUCTURAL METAL MEMBERS WITH A IMPROVED FRACTURE TOUGHNESSFiled June 14, 1965 In. Lbs/In FRACTURE TOUGHNESS ZNVENTOR. HERBERT L.LE/CHTER ATTORNEY United States Patent 3,359,083 COMPOSITE STRUCTURALMETAL MEMBERS WITH IMPROVED FRACTURE TOUGHNESS Herbert L. Leichter,Castro Valley, Califi, assignor to the United States of America asrepresented by the United States Atomic Energy Commission Filed June 14,1965, Ser. No. 463,973 8 Claims. (Cl. 29-1835) ABSTRACT OF THEDISCLOSURE High fracture toughness laminate comprised of superposedsheets of metal joined together, each sheet having a thickness which issubstantially equal to that at which the fracture toughness of thespecific material of which the sheet is made is at a maximum.

The present invention relates to laminar structural materials, and moreparticularly to laminates having a predetermined laminar thickness tooptimize the fracture toughness thereof.

The fracture toughness of a material is a quantity which is related tothe force necessary to induce growth of a fracture or flaw to such anextent that failure of the material structure occurs. The stressesnecessary to produce failure in structural materials having flaws,cracks, or similar imperfections are generally considerably less thanthe inherent ultimate or yield strength of the material.

The present invention provides a laminar structural material whichexhibits enhanced fracture toughness and which is, therefore, highlyresistant to failure due to crack propagation at stresses below theinherent or ultimate yield strength. The present laminar material iscomprised of a multiplicity of joined laminae, the thickness of eachpreferably being selected to be near the thickness at which the fracturetoughness is at a maximum and the selected laminae are then bonded toprovide a composite structure in which the fracture or stress failureresistance of the composite structure is optimized.

In mathematical terms, the fracture toughness of a material may becharacterized by the expression where E is the modulus of elasticity ofthe material, 1r is the universal constant for a circle, and Grepresents the stored elastic strain energy which is released as a crackof length a advances over an additional unit area of the material if thematerial is under the influence of a stress 0' normal to the plane ofthe crack. In the limit, G approaches a critical value G, which isdefined as the release of an amount of energy which in response to theloading of the material leads to immediate failure by rapid crackpropagation. This value is commonly known as the fracture toughness ofthe material. The fracture toughness of a material appears to be a basicmaterial property, and is a function of a number of variables, inparticular the structure and composition of the material, thetemperature of the material, the rate of loading, and the dimensions ofthe material.

Most important from the point of view of the subject of the presentinvention, the fracture toughness of cer- 3,359,083 Patented Dec. 19,1967 tain laminar materials has been found to be a function of thethickness dimensions of a homogeneous specimen of material in which acrack may be located. Further, it has been discovered that thefunctional relation between fracture toughness and the thickness ofmaterials exhibit a maximum over readily ascertainable thickness ranges.In the case of metals, it has been confirmed by experimental testingthat the maximum generally occurs at a thickness of less than about 0.2inch and down to about 0.01 inch. Moreover, it has been found that sheetor laminar materials, e.g. metal sheet, having a thickness dimensionnear that at which fracture toughness is a maximum can be bonded inintimate laminar relationship to yield a composite structural bodyhaving a fracture toughness which greatly exceeds what might beexpected. The reason why laminar or sheet materials should exhibit amaximum fracture toughness related to thickness is not completelyapparent. It is possible that the method used to produce the sheetmaterial, e.g. rolling or other mechanical working with intervening heattreatment which necessarily varies with the thickness of the sheet beingformed yields a metallographic structure at certain thicknessesconnoting a particular treatment which yields a metallurgical structureof optional properties. Moreover, minute or incipient fracture defectsmay have a minimum occurrence in said range. Whatever the cause laminarmaterials exhibiting such maximal fracture toughness can be fabricatedinto composite structural bodies of exceptional toughness in accordancewith the invention. For example, a titanium aluminum tin alloy compositelaminate having a thickness of 0.4 inch and comprised of individuallaminae having a thickness of 0.062 inch was found to have a fracturetoughness for through-the-thickness cracking up to 7.8 times as great asthat of a homogeneous body of the same material of equivalentdimensions. In the present context where even a doubling of fracturetoughness would be of significant interest, the results obtained inpractice are remarkable.

In the prior art materials which have anisotropic strength propertieshave been incorporated into laminar structure to provide high strength.For example, various textile and fibrous materials are strengthened inthe direction perpendicular to the fibers by joining together layers inwhich the fibers are alternately perpendicular or angularly disposed toeach other, i.e. with respect to the direction of maximum textilestrength. Laminating procedures have also been used to join togethermaterials having different properties in order to obtain a compositewhich has the combined properties of the constituent materials.Generally, fiexural modulus is enhanced but the tensile strength isalmost completely determined by the additive contributions of individualfibers.

As distinguished from these prior art laminates, the present inventionprovides a laminar body of improved fracture toughness due to thecontrolled laminar thickness. It should be understood that theimprovement in fracture toughness can be achieved for materials havingisotropic as well as anisotropic characteristics, whether theorientation of the material in successive layers is identical or indifferent directions. Accordingly, the present material may be thoughtof as being in the nature of a continuous structural body, which is acomposite of laminae of selected thickness dimensions related to amaximum in the fracture toughness characteristic curve joined or bondedin face to face laminar relation to provide a composite structuralmember of exceptional fracture toughness.

The fracture toughness of a material has been shown to be a quantitywhich is extremely important in practical engineering work. In practice,many structural materials are likely to have flaws or cracks as aconsequence of which structural failure is likely to occur due to crackpropagation at loads considerably and unpredictably less that theultimate strength of a material as determined from tests of a nearperfect specimen. To allow for the unpredictable deleterious influenceof cracks and flaws on the strength properties of materials, structuresare usually designed with relatively large safety factors. In addition,structural materials are generally tested and inspected to locate cracksand flaws by procedures which are time-consuming and costly. Thereafter,appropriate remedial action may be taken, such as welding, or evendiscarding the member entirely at considerable economic loss. Obviously,to locate smaller imperfections in the interest of increased safety, theinspection procedures grow more elaborate and time consuming. It isevident therefore that the use of conventional structural materialsinvolves a safety or weight penalty which is a serious dis-advantageespecially in aerospace applications, in the construction of submarinehulls, or any other application, where a structure is exposed to highand repetitive loading or high impulse shock loading conditions. Forsuch applications, the laminate of the present invention is ideallysuited by virtue of its improved fracture toughness characteristics.

Accordingly, in summary, the principal objects or advantages of thepresent invention are:

(1) To provide a structural material having greatly improved fracturetoughness without increasing size or impairment of other physicalproperties;

(2) To diminish the necessity for rigorous inspection of the materialfor small order defects by providing a structural material which is lessliable to fracture due to crack propagation;

(3) To provide a material especially suited for use where impulse shockloading is encountered; and

(4) To provide a laminated configuration for structural members which ischaracterized by an inherently high margin of safety against failure dueto defect growth as compared to similar and continuous homogeneousstructural members of isotropic composition.

Other objects and advantages will become apparent to those skilled inthe art upon consideration of the following description and accompanyingfigures of which:

FIGURE 1 is a graph of the fracture toughness of various metals asrelated to the thickness of the material.

FIGURE 2a is an isometric drawing of a laminated structural testspecimen which is notched and fatigue cracked across the face of the toplamina.

FIGURE 2b is an isometric drawing of a laminated structural testspecimen which is notched and fatigue cracked through-the-thicknessperpendicular to the plane of the laminae.

In general, as mentioned above, the present invention is a compositelaminated structural material formed of bonded laminar or sheetmaterials which is characterized by the property that its fracturetoughness varies with the thickness of the material and exhibits amaximum. The laminate is comprised of a multiplicity of individuallaminar layers, the thickness of each being selected at or near thethickness corresponding to the maximum fracture toughness. The laminarlayers are joined together in face to face relation into an integralbody having the overall dimensions of a desired structural member.

For practical application, the present laminates are used in themanufacture of structural beam members or plate, the total thickness ofwhich exceeds the optimum fracture toughness of the material by at leasta factor of about two.

However, it is noted, that the failure resistance of a structuralmaterial is increased whenever the weighted average of the fracturetoughness of the individual laminae exceeds the fracture toughness of acontinuous homogeneous body of the same material as said laminae. Due tothe properties of the composite laminar structure the toughnessproperties of structural bodies fabricated in this manner are with greatreliability in excess of the equivalent unlaminated conventionalstructures while other advantages, e.g. reduced weight can often beattained since a considerably reduced safety margin can be reliablyused.

To obtain a composite which is characterized by an enhanced fracturetoughness compared to a continuous body of the same dimensions, e.g.produced by molding, casting, forging, machining as one solid piece, thelaminae of appropriate shape may be joined by any bonding method,including adhesive and brazing or by mechanical means, such asspotwelding, riveting, or parts may be punched from laminates, in someinstances. For structural materials, however, it is preferred to achievedistribution of the stresses over the entire member and for structuralpurposes, the laminae preferably are joined in face to face relation byusing an appropriate bonding medium to render an integral or unitarybody where the bond between the laminae extends uniformly over theentire faces of the laminae. Forces exerted on such a body, e.g. shearstresses, etc., are thereby transmitted between the laminae and evenlydistributed over the entire structural member to provide bodies of highflexural modulus, strength, etc., while simultaneously obtaining thegreatly enhanced fracture toughness.

Referring now to FIGURE 1, graphs are presented illustrating thefracture toughness-thickness relation of two alloys. The graph labelled(a) is for a titanium alloy, specifically Ti-5Al-2.5Sn, and the graphdesignated (b) is for grade 300, 18% Ni maraging steel. Both of thesecurves are seen to exhibit a maximum fracture toughness in the thicknessregion between 0.025 and 0.1 inch. The fracture toughness decreases withincreasing thickness of the sheet or member and asymptoticallyapproaches a constant value. The fracture toughness of a continuoushomogeneous material is equal to the fracture toughness corresponding toits thickness as given by the graph. The fracture toughness of alaminate, however, is deter-mined in large measure by the selectedthickness of individual laminae providing toughness of the individuallaminae, and further by the combination of selected laminae to provide acomposite structural body. If the individual laminae have a thicknessequal to the thickness at which the fracture toughness of the materialis a maximum, the laminate comprised of such laminae will have afracture toughness which is significantly superior to any continuoushomogeneous body of equal or even greater total thickness constructed ofthe same material.

To determine the fracture toughness of a material, several methods maybe employed. Tests performed at slow strain rates are the Center-NotchTensile Test and the Slow-Bend Prenotched Charpy Method. These methodsare described in detail in the following references: American Societyfor Testing Material (ASTM) Committee Reports on Fracture Testing ofHigh Strength Metallic Materials, ASTM Bulletin, January and February1960; and Sheet Fracture Toughness Evaluation by Impact and Slow Bend,G. M. Orner and C. E. Hartbower, Welding Journal, Research Supplement,September 1961.

Another method for determining the fracture toughness of materials isthe Prenotched Charpy Impact test, which is carried out at high strainrates of about 10 in./in. sec. This test has limitations the valuesobtained for very tough materials are generally low. While measuringabsolute fracture toughness values is somewhat subject to error, thePrenotched Charpy Impact Test yields a fracture toughness-thicknesscurve with a reliable shape and relative values. However, for presentpurposes the primary purpose is to determine the material thickness atwhich the fracture toughness is a maximum and to obtain a knowledge ofthe relative increase of the fracture toughness which can be achieved bya laminate over a continuous homogeneous body. Since this informationcan be obtained from the test with reasonable accuracy the impact testsat high strain rates are preferred because of the relative speed andeconomy for preparing and testing large numbers of specimens. Theprocedure is described in detail by Hartbower and Orner in MetallurgicalVariables Affecting Fracture Toughness in High Strength Sheet AlloysTechnical Documentary Report No. ASDTDR62-868, June 1963. The PrenotchedCharpy specimens are fatigue cracked at the base of the notch with afatigue cracking machine. The application of stress is continued untilthe cracks become about 0.025 to about 0.035 inch deep. The variation ofthe depth of the fatigue cracks within these limits does notsignificantly affect the results of the tests. The fracture toughness isdefined as the energy absorbed in fracturing the specimen, divided bythe area of the fracture. This value is determined for a number ofspecimens of varying thicknesses and plotted. A smooth curve joining theindividual measurements will then result in a graph as illustrated inFIGURE 1. Of particular interest for purposes of the present inventionare portions of enhanced fracture toughness of the curves (a) and (b) inthe thickness ranges R and R for the titanium alloy and the steel. Thefracture toughness of the material having a thickness in this rangesubstantially exceeds the fracture toughness values G which the curveapproaches.

For purposes of the present invention and practical application, theregion of enhanced fracture toughness is defined as extending betweenthe thickness values at which the fracture toughness is intermediatebetween the maximum fracture toughness and G Referring now to FIGURES 2aand 21), there are shown laminated structural beams constructed inaccordance with the present invention. Individual laminae 11 are joinedtogether in face to face relationship. The thickness d of each laminacorresponds to the thickness of the sheet material under the elevatedportion of the fracture toughness curve shown in FIG. 1. The joints 12are formed most commonly and preferably by a layer of metallic bondingor joining alloy, e.g. brazing material fused to the faces of adjacentlaminae 1 1. The bond formed between the braze material and the laminae11 extends continuously over the entire face of the laminae. The brazematerial is selected in accord with usual engineering practice on thebasis of the quality of the bond which the braze forms with the laminamaterial, and on the basis of its physical properties which must becommensurate with the environmental demands on the laminate. While thepreferred method of bonding the laminae 11 is brazing, which will betaken to include all appropriate methods using a bonding alloy or adiffusion bonding agent, e.g., silver, it will be realized, that whenthe environmental conditions permit their use, other joining methodsmaybe used in place of brazes, provided that a firm bond is formedbetween the laminae 11. Thus, in low temperature applications requiringonly moderate bonds strength, synthetic adhesives such as catalyzedepoxy adhesive resins can be used to join laminae 11. With brazing ordiffusion bonding, the present laminates are constructed by heating a.pressurized stack of alternate layers of brazing foil and laminar platesto the brazing temperature of the foil, preferably in an inertatmosphere.

A preferred method of making the laminate is by the Hortonclad process,described in detail in US. Patent No. 2,713,196 issued to R. L. Brown onJuly 19, 1955. According to this method material sheets are stacked withbraze alloy in foil form interposed between the sheets. This stack ofalternating laminar material and braze alloy foil is sealed in aflexible steel envelope, placed under a vacuum, and heated to thebrazing temperature and held at this temperature for a specified perioddepending on the brazing material.

A preferred method to carry out the brazing step, which is especiallysuitable for brazing in small lots, where less than one atmosphere ofpressure is required, is to dispose the stack of laminar material andinterposed braze foil in a stainless steel envelope. This envelope ishermetically sealed and adapted for evacuation. Prior to disposing thestack in a furnace, the envelope is evacuated to a pressure of the orderof microns. The stainless steel envelope collapses and compresses thestack, while excluding most of the air from the stack during the heatingstep. The envelope is then heated to and held at the brazing temperaturein a furnace for a specified period. The composite laminate of theinvention and manufacturing method will be further illustrated in thefollowing specific examples.

EXAMPLE I.Ti-5Al2.5Sn LAMINATE The fracture toughness-thickness relationfor the titanium alloy Ti-5Al-2.5Sn laminate was determined as outlinedin the description above. The graph was found to exhibit a maximum inthe thickness region at about 0.075", as illustrated in FIG. 1a.Accordingly, a laminate plate Was fabricated using titanium alloy sheetof a thickness 0.062 in. which falls within the enhanced fracturetoughness region. A stack of 6 alloy laminae was assembled withcoextensive sheets of braze alloy foil interposed therebetween. Thepreferred braze alloy composition was 92% Ag, 7.5% Cu and .5% Li, andthe thickness of the foil sheets was 2 mils. The stack was placed into astainless steel envelope and compressed by evacuating the envelope toabout microns. The envelope was placed into a furnace and the compositewas heated to a temperature of 1727:2 F. This temperature and thepressure of 150 microns were maintained for a period of about 5 minutesto produce a brazing joint between the titanium alloy laminae.Thereafter the envelope containing the laminate was air cooled undervacuum. The integrity of the bonds between the laminae was checked fordefects by ultrasonic methods.

Charpy specimens were cut from the laminated plate, notched and fatigueprecracked to a depth of about 0.03" and the properties tested by thePrenotched Charpy Impact Test described above. With reference to FIG.2a, one type of specimen was notched across the face 13 of the top layernormal to the plane of the lamina. A second type of specimen,illustrated in FIG. 2b was notched across the sides 14 and athrough-the-thickness fatigue crack introduced at the bottom of theV-shaped notch. The physical properties of both laminar specimens andidentical specimens of a continuous forged bar were tested under thesame conditions. The laminates which were notched and cracked across theface of the top layer could not be completely broken at the maximumimpact loading of 240 ft. lbs. delivered by the pendular hammer of thetesting machine. The results of the tests with the through-the-thicknesscracked specimen and the continuous bar are given in Table 1.

EXAMPLE II.GRADE 250 18% NICKEL MARAGING STEEL 250 grade maraging steelexhibits maximum in the region between about 0.02 and 0.08 inch. Asix-ply laminated plate of grade 250 18% nickel maraging steel was madeby assembling a stack of 6 steel laminae of a thickness of 0.062 inch. A2.0 mil thick braze alloy foil having a composition of 92% silver, 7.5%copper and 0.5% lithium was placed between successive steel sheets. Theassembly was disposed into a flexible steel envelope, placed into afurnace and heated to a brazing temperature of 17551-5 F. for a periodof 10 minutes, while maintaining the vacuum at about 40 microns. Thelaminate was then air cooled and reheated to the anstenitizingtemperature of 1500 F. After 40 minutes the laminate was again aircooled. Notched Charpy structural members were prepared from thelaminate and from homogeneous maraging steel bar stock and tested asoutlined above. The results are also given in Table I. Again thespecimens notched across the face of the top layer exceeded the 240 ft.lb. capacity of the testing equipment. Rough calculation based on thepartially fractured specimens indicated a fracture toughness betweenabout 30 and 40 times as great as the fracture toughness of thecontinuous bar.

URE TOUGHNESS OF LAMINA TABLE I. FRACT PLATE MATERIALS AT R TES, SINGLESHEET, AND CONTINUOUS M TEMPERATURE Fracture Yield Ultimate Elon-Material Constitution of thickness Toughness Strength, Strength, gation,in lb .lin. lh./in.'- lb./1n. percent Ti, SAl, 2.5 Sn AMS 4910 .374"plate l 517 130, 300 132, 300 21. 0 Ti, SAl, 2.5 Sn AMS 4010"... .062sheet 1 5, 410 115,000 120, 700 21. 8 Ti, SAl, 2.5 Sn AMS 4910". .002Sheet 1 4, 540 115,000 120, 700 21, 8 Ti, SAl, 2.5 Sn AMS 4910. 6 plylaminate of .062 sheet.. 1 4, 046 115, 000 120, 750 19. Ti, SAL 2.5 SnAMS 4910". 2 4, 040 115, 000 120, 750 19. 5 574 3 271, 400 3 279, 600 311. 0 5 403 3 271, 400 3 279, 600 3 11. 0 1 1,200 3 274,300 3 287, 300 33. 2 .do 2 1,204 3 274, 300 3 287, 300 3 3, 2 0 ply laminate of .062Sheet 1 1, 045 3 264, 000 3 276, 000 3 8. do 3 800 3 204, 000 3 276,0003 8. 25

1 Perpendicular to rolling direction 0f material. 2 Parallel to rollingdirection of material. 3 Values given are for 300 grade steel.

The data in- Table I illustrates the improvement in the fracturetoughness of the laminated materials over the continuous homogeneous barmaterials. Although only two specific examples have been given, it isnot intended to convey that the invention be limited to these specificmaterials. The remarkable improvement of the fracture toughnesscharacteristic of these laminates over homogeneous bodies of the samedimensions can be analogously achieved in other materials, provided onlythat the fracture toughness-thickness curve for the material exhibit amaximum and that a suitable binder be employed to join together thelaminar composite. Moreover, while the optimum or maximum effects aregenerally obtained using a laminate comprised only of metal sheetshaving the specified maximum fracture resistant thicknesses improvementis obtained if even a single such sheet is used with other thicknesssheets and with sheets of other materials laminated therewith. Theadditional benefits obtained in the composite structure generallyrequires that two or more such sheets or laminae be used. Therefore, thescope of the invention is to be limited only by the following claims.

What is claimed is:

1. A composite structural body of improved fracture toughness,comprising: a plurality of laminar sheets of a titanium alloy consistingessentially of 92.5% by weight of titanium, 5% by weight of aluminum and2.5% by weight of tin, a combined thickness substantially greater thanthe thickness range in which the fracture toughness of a homogeneousbody of said alloy is at an optimum, said sheets having a thickness inthe range of 0.02 and 00.2 inch wherein the sheets individually exhibitan optimum fracture toughness, said laminar sheets being disposed incontiguous face-to-face relation, bonding means joining said laminarsheets in said contiguous face-to-face relation to provide a structuralbody of improved fracture toughness.

toughness of said steel is at a maximum, said sheets individually havinga thickness in the range of about 0.01 to 0.15 inch wherein said steelexhibits an enhanced fracture toughness, said laminar sheets beingdisposed in contiguous face-to-face relation and bonding means joiningsaid laminar sheets in said contiguous face-to-face relation to providea structural body of improved fracture toughness.

4. The structural body of claim 3 further defined in that said bondingmeans is a bonding agent layer disposed between and fused to adjacentfaces of said laminar sheets.

5. The laminate of claim 1 further defined in that said titanium alloylaminar sheets have a thickness in the range of about 0.03 and 0.13inch.

6. The laminate of claim 2 further defined in that said bonding agentlayer is a brazing alloy consisting of 92% by wt. of silver, 7.5% by wt.of copper, and 0.5% by wt. of lithium.

7. The laminate of claim 3 further defined in that said maraging steelsheets have a thickness in the range of about 0.02 and 0.08 inch.

8. The laminate of claim 4 further defined in that said braze jointlayer is comprised of a brazing alloy consisting of 63% by wt. ofcopper, 22% by wt. of manganese, 10% by wt. of cobalt and 5% by wt. ofnickel.

References Cited UNITED STATES PATENTS 125,245 4/1872 Absterdam 29196.1365,300 6/1887 Farrel 29-198 X 1,005,142 10/1911 Becker 29191.6 X1,087,561 2/1914 Tebbetts 29-194 2,438,759 3/1948 Liebowitz 29l96.12,767,467 10/1956 Sicgel 29-194 2,908,969 10/1959 Wagner 29-198 X2,713,196 7/1956 Brown 29-196 HYLAND BIZOT, Primary Examiner.

3. A COMPOSITE STRUCTURAL BODY OF IMPROVED FRACTURE TOUGHNESS,COMPRISING: A PLUURALITY OF LAMINAT SHEETS OF MARAGING STEEL OF ACOMBINED THICKNESS SUBSTANTIALLY GREATER THAN THE THICKNESS RANGE INWHICH THE FRACTURE TOUGHNESS OF SAID STEEL IS AT A MAXIMUM, SAID SHEETSINDIVIDUALLY HAVING A THICKNESS IN THE RANGE OF ABOUT 0.01 TO 0.15 INCHWHEREIN SAID STEEL EXHIBITS AN ENHANCED FRACTURE TOUGHNESS, SAID LAMINARSHEETS BEING DISPOSED IN CONTIGUOUS FACE-TO-FACE RELATION AND BONDINGMEANS JOINING SAID